This patent application is related to a patent application filed by the same applicant on even date herewith and having an internal reference of A30748, and claiming priority from European Patent Application 06251879.0.
The invention relates to autonomous networks of wireless devices, and has been developed for use in sensor networks for monitoring environmental conditions in situations where it is not practical to provide a fixed infrastructure, such as oceanography. However, the invention is also applicable in other fields in which wireless devices are free to move relative to each other, and there is little or no fixed infrastructure. Example applications include battlefield use, disaster recovery, and inter-vehicle communication (“traffimatics”).
The wireless devices have numerous conflicting requirements, particularly as regards prolonged battery life and reliability and speed of the communication of data to a data collection station. In general, communication reliability and data relay speed can be improved by increasing transmitter power and the frequency at which data is transmitted, although these measures increase power consumption and thus reduce battery life.
Furthermore, the provision of wireless devices which are free to move, for instance on ocean currents, provides challenges since the devices can then move relative to one another, giving rise to changes in network topography over time. This is particularly problematic when the devices are used as relays to pass data towards a base station from devices too remote to access the base station directly since the reliability, and indeed existence, of communication paths is sensitive to the correct operation of every device in the path and to the reliability of the links between each device and the devices immediately upstream and immediately downstream of it in the path. The reliability of a link can be compromised by environmental conditions, for instance the presence of waves or land masses, as well as by the physical distance between the devices at the ends of the link.
The present invention was made in this context. According to a first aspect of the invention, there is provided a method of operating a plurality of wireless devices each including a receiver and a transmitter, the method comprising:
controlling each device to use received signals to divide time into a succession of frames, each frame having plural consecutive timeslots, and into a succession of superframes, each superframe comprising at least four frames;
wherein the frames represent different levels in a hierarchy of the devices;
operating the transmitter of each device in one or more timeslots of a first frame over each of a plurality of superframes, and refraining from operating the transmitter in the other frames of the superframes;
operating the receiver of each device in at least some of the timeslots of at least the two frames in each superframe representing the levels in the hierarchy adjacent to the level represented by the first frame; and
refraining from operating the transmitter and the receiver in at least one further frame in the superframe.
This invention allows a device to be associated with a hierarchy of devices in which devices at the same level in the hierarchy transmit in the same frame as each other and do not transmit in any other frames.
Each superframe comprises at least four frames, three of which represent, for each device, its own level in the hierarchy and the two adjacent levels The allocation of individual frames in the superframe to relative levels in the hierarchy should be consistent, so that the frame management of each device is consistent across all levels in the hierarchy. Thus a device, having identified its position in the hierarchy relative to its neighbours, can readily identify the first (transmit) frame and the two frames allocated to the adjacent levels. Although other distributions are possible, placing frames relating to adjacent levels in a hierarchy adjacent to one another in the superframes makes frame management in each device straightforward, and allows frames to be used by devices which are not at the same level in the hierarchy.
It is not necessary to define an absolute level. The three frames in question will differ from one level in the hierarchy to the next, but may be re-used at sufficiently higher or lower levels—a theoretical minimum of four levels is necessary in each superframe, but there are preferably between 10 and 50. Devices which are at levels in the hierarchy separated by a number equal to the number of frames in a superframe have transmit slots in the same frame. However, since in practice it is unlikely that such devices would be in sufficiently close geographical proximity to interfere with one another, the inventor considers that the bandwidth saving provided by this outweighs disadvantages potentially arising from such interference.
By operating the receiver in frames other than the frame in which the device transmits, the device can gain information about neighbouring devices at different levels. The information can be such as to allow it to make decisions about data routing, etc. Significantly, this invention provides the device with one or more frames in which it is not required to transmit or to receive, so allowing predictable hibernation or standby of the device or components thereof, or the use of those components for other purposes, for instance sensing or data processing, without impinging on the ability of the device to operate its transmitter and receiver at the appropriate times.
In this way, the device can receive data from a device at a greater hop distance from a base station and can selectively forward data to devices at a lower hop distance from the base station. This can allow devices to effect intelligent data passing between devices at different levels of the hierarchy.
The method may comprise operating the receiver over plural superframes in timeslots of the first frame in which the device does not operate the transmitter. This allows a device to obtain information about neighbouring devices which are at the same level as it. With suitable provision in the device, this information can be used to improve the effectiveness, reliability and efficiency of a system in which the device is incorporated. In this case, the method may comprise operating the receiver in at least some of the timeslots of the first frame in which the device does not operate the transmitter. This allows a device to obtain information about a maximal number of neighbouring devices which are at the same level as it.
Preferably, all the devices operate at the same frequency.
Optionally, on correctly receiving a packet in a timeslot, a code of the received packet is separated from payload data to provide a received code and the received code is stored according to the timeslot in which it was received. The timeslot in which the packet is received can be associated with a particular device. Advantageously, the reliability of communications between the receiving device and the device which transmits during that transmit timeslot can be determined based upon the number of codes stored in respect of a particular timeslot. In this case, it can be advantageous for the method to comprise comparing the bits of the received code that relate to the timeslot in which it was received to corresponding bits of a code generated by the device relating to the same timeslot. Thus, the device is able to compare the reliability of links between other devices with links between the device and those other devices.
Some or all of the devices may include a scheduling function operable to control the device to perform calculations and other functions at times other than times when the transmitter and the receiver are in use. This allows a simpler form for the hardware, such as a controller, to be used, which can have savings in terms of cost and/or power consumption. Alternatively or in addition, the scheduling function may be operable to control the device to operate a sensor to perform a sensing operation at times other than times when the transmitter and the receiver are in use. Alternatively or in addition, the scheduling function may be operable to control the device to hibernate between the frames in which the transmitter and the receiver are in use in two successive superframes.
Any of the methods may be implemented by a computer program, which may be encoded on a carrier capable of being loaded into a suitable general-purpose computer.
According to a second aspect of the invention, there is provided a wireless communications device including a receiver and a transmitter, the device being arranged to:
use received signals to divide time into a succession of frames, each frame having plural consecutive timeslots, and into a succession of superframes, each superframe comprising at least four frames;
wherein the frames represent different levels in a hierarchy of the devices;
operate the transmitter in one or more timeslots of a first frame over each of a plurality of superframes, and refraining from operating the transmitter in the other frames of the superframes;
operate the receiver in at least some of the timeslots of at least the two frames in each superframe representing the levels in the hierarchy adjacent to the level represented by the first frame; and
refrain from operating the transmitter and the receiver in at least one further frame in the superframe.